The present invention relates to novel squarylium compounds which can be used in the optical recording field, and to optical recording media using the same.
In recent years, development of a digital versatile disc-recordable (DVD-R) as a recordable optical recording medium having a higher recording density than that of a compact disc-recordable (CD-R) has been under. Both of CD-R and DVD-R are similar to each other in that an organic dye is utilized therein as a recording material and in a principle of recording and reproducing of a signal (information). Therefore, the organic dyes developed for CD-R can basically comply with the various requirements (light resistance, solubility, thermal decomposition properties) for the recording material of DVD-R other than spectroscopic properties. However, an oscillation wavelength of a semiconductor laser, which is used for recording the signal to DVD-R or for reproducing the signal from DVD-R, is in the range of 600-700 nm, which is shorter than that of the semiconductor laser which is used for CD-R. Accordingly, the recording material utilized for DVD-R should have an absorbance end of a longer wavelength side shorter than that of CD-R when it exists in the form of a membrane. Therefore, the dyes developed for CD-R such as cyanine dyes, azaannulene dyes and indoaniline-metal chelate dyes (xe2x80x9cElectronics Related Dyesxe2x80x9d, CMC, 1998) can not be used as the recording material for DVD-R.
The present inventors have developed squarylium compounds having different two kinds of aromatic substituents in a molecule. Such squarylium compounds have a squaric acid skeleton at a center of the molecule and substituents comprising an aromatic compound on carbon atoms at two catercornered positions of the skeleton. Squarylium compounds having two same aromatic substituents are conveniently referred to as symmetric squarylium compounds (or symmetric squarylium dyes), whereas those having different two kinds of substituents are referred to as asymmetric squarylium compounds (or asymmetric squarylium dyes).
The symmetric squarylium compounds having the same two pyrazole structures in the molecule have been already known (DE 2055894). In addition, some kinds of asymmetric squarylium compounds having one indoline structure and another aromatic substituent different from the indoline structure in the molecule have been known (Japanese Unexamined Patent Publication No. 339233/1993). However, asymmetric squarylium compounds having only one pyrazole structure, or having one pyrazole and one indoline structures have not been known yet.
In view of an oscillation wavelength of the semiconductor laser used for DVD-R, for spectroscopic properties of the recording material, which have the close relation with recording and reproducing sensitivities of the signal, it is desirable that the maximum absorption wavelength (xcexmax) of the recording material measured in its liquid state is within the range of 550-600 nm and log xcex5 thereat (xcex5 is a molar extinction coefficient) is 5 or greater. In addition, for thermal decomposition properties of the recording material, which have the close relation with the recording sensitivity, it is desirable that a significant loss in weight is observed within the temperature range of 250-350xc2x0 C.
Furthermore, although light resistance and solubility in a solvent which is necessary for membrane formation are also required as the property of the recording material, there is no recording material having suitable properties for DVD-R, such as spectroscopic properties, light resistance, solubility and thermal decomposition properties, in the known squarylium compounds.
An object of the present invention is to provide squarylium compounds having spectroscopic properties, light resistance, solubility and thermal decomposition properties suitable as a recording material for DVD-R, and optical recording media using the same.
In view of the above situation, the present inventors intensively investigated and, as the result, we successfully synthesized novel asymmetric squarylium compounds having pyrazole and indoline skeletons in the molecule, and obtained a finding that such squarylium compounds have properties suitable as a recording material for DVD-R.
The present invention was done based on such a finding, and provides squarylium compounds represented by the formula (I): 
wherein, R1 represents a hydrogen atom, an alkyl group optionally having a substituent, an aryl group optionally having a substituent, or a heterocyclic group optionally having a substituent; R2 represents a hydrogen atom, a halogen atom, an alkyl group optionally having a substituent, an alkoxy group optionally having a substituent, an aralkyl group optionally having a substituent, an aryl group optionally having a substituent, an amino group optionally having a substituent or a heterocyclic group optionally having a substituent; R3 and R4 are the same or different, and represent an alkyl group, or R3 and R4 may be taken together with an adjacent carbon atom to form an alicyclic hydrocarbon ring or a heterocycle; R5 represents a hydrogen atom, an alkyl group optionally having a substituent, an aralkyl group optionally having a substituent or an aryl group optionally having a substituent; R6 represents a halogen atom, an alkyl group optionally having a substituent, or an aralkyl group, an aryl group, a nitro group, a cyano group or an alkoxy group; and n represents an integer of 0-4, and when n is 2-4, then R6s are the same or different, or two adjacent R6s may be taken together with two adjacent carbon atoms to form an aromatic ring optionally having a substituent, and optical recording media which has a recording layer comprising said squarylium compound.